Some common types of high intensity discharge lamps used for lighting comprise a quartz or fused silica arc tube enclosed within a glass outer jacket fitted with a screw base at one end. In high pressure mercury vapor lamps the arc tube contains a filling of mercury, whereas in high pressure metal halide lamps, the arc tube contains a filling of mercury and metal halides. In both kinds, the inner arc tube transmits ultraviolet radiation which is absorbed without harm by the glass outer envelope, or even absorbed gainfully by a phosphor coating on the outer envelope.
In most lamps the outer envelope remains intact to the end, and life is ended by other factors. However it does happen occasionally that the outer envelope or glass jacket is shattered and the arc tube remains intact so that the lamp may continue to operate. In this mode of operation, the ultraviolet radiation from the arc tube is not restrained and may create a safety hazard.
It has been proposed to provide a lead wire fuse of a metal which oxidizes rapidly upon contact with air. Such fuse located in the outer envelope burns up when air enters and disables the lamp. However, the continuous ohmic loss due to current flow through the lead wire fuse entails a substantial reduction in overall efficiency, particularly in lamps of lower wattage, and a better solution is desired.